The present invention relates to a DC rotary electric machine of permanent magnet field type, or more in particular to a structure of the field magnets with an improved stator.
In the conventional DC rotary electric machines of permanent magnet field type, as disclosed in JP-A-57-153558, as many field magnets as the magnetic poles of a rotary electric machine are arranged on the inner periphery of the stator, and each of the field magnets includes a permanent magnet and an auxiliary magnetic pole of a magnetic material such as mild steel juxtaposed with the permanent magnet.
The conventional DC rotary electric machine of permanent magnet field type, however, with a permanent magnet arranged with each of the four magnetic poles, develops a great amount of magnetic flux at the permanent magnets under a no load, thereby resulting in a low no-load speed of rotation. Further, since each permanent magnet is made of a rare earth material such as samarium, cobalt or Nd-Fe-B, the great amount of material required leads to a high cost of the field magnets. In addition, when assembly bolts are passed through the space between adjacent magnetic poles for assembly work, each bolt, being of a magnetic material, is attracted to the permanent magnets, thereby making the assembly work inconvenient.